In network systems in FA (Factory Automation), one or a plurality of PLCs (Programmable Logic Controllers) that control input devices and output devices of industrial robots and other manufacturing equipment installed in manufacturing plants and devices that are controlled by the operation of the PLC are connected to a network of a control system.
A PLC is configured by coupling various types of units, such as a CPU unit and I/O units. Examples of I/O units are input units that are connected to sensors, switches or other input devices and take in their on/off signals as input signals, and output units that are connected to actuators, relays or other output devices and send out output signals to them. Based on output data that the MPU of the CPU unit has written into a memory of the output units, the output units output the output signals to the output devices. The input signals that are input into the input devices are stored in a memory within the input unit and the MPU of the CPU unit accesses this memory to obtain input data. Thus, the MPU of the CPU unit reads and writes data by accessing the memories of the individual units. However, as the processing speed increases, accessing the memories of the various units with the MPU to read and write data becomes difficult.
To address this problem, the present inventors have conceived the exchange of data between the units constituting a single PLC, utilizing communication technology between nodes connected to a PLC network cable. More specifically, data is exchanged by master-slave communication with the CPU unit as a master and the other units as slaves. Moreover, in this kind of communication technology, a specification is adopted in which an ACK frame is sent back from the recipient when a transmission frame has been properly received by the recipient, and if the ACK frame is not returned within a predetermined period of time, then the master judges that their delivery has failed and the frame is resent. Thus, the intended data can be reliably delivered to the recipient. This kind of resend technology is disclosed in Patent Document 1, for example.